Slice multiplexing method and apparatus for magnetic resonance imaging

ABSTRACT

In a magnetic resonance slice multiplexing method and apparatus, measurements are performed repeatedly subject to the assignment of additional phases to the respective slices, the additionally assigned phases being changed with reach repetition such that at least one central k-space region is sampled completely in each of the repeated acquisitions. A calibration dataset is determined from the measurement data acquired completely in the central k-space region. The calibration dataset is used when reconstructing image data for the simultaneously excited slices from the acquired measurement data.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention concerns a method for generating measurement datafor at least two non-overlapping slices of an examination object bymagnetic resonance (MR) technology using an MR data acquisitionsequence, which determines measurement points on a k-space trajectory,and has at least one multiband RF excitation pulse, which excites the atleast two non-overlapping slices simultaneously and selectively.

Description of the Prior Art

Magnetic resonance (MR) technology is a known modality, used to generateimages of the interior of an examination object. In simple terms, tothis end the examination object is positioned in a magnetic resonancescanner in a powerful static, homogeneous, constant magnetic field, alsoreferred to as the B₀ field, with field strengths of 0.2 tesla to 7tesla and more, so that nuclear spins in the object are oriented alongthe constant magnetic field. To trigger nuclear magnetic resonance,radio-frequency excitation pulses (RF pulses) are radiated into theexamination object so as to trigger magnetic resonance signals that aredetected and entered as complex numbers into a memory are referred to ask-space data, and MR images are reconstructed or spectroscopy data aredetermined from k-space data. To spatially encode the measurement data,the constant magnetic field is overlaid with rapidly switched magneticgradient fields. The recorded measurement data are digitized and storedas complex numerical values in a k-space matrix. An associated MR imagecan be reconstructed from the value-populated k-space matrix, forexample by means of a multidimensional Fourier transform.

In order to improve the signal to noise ratio (SNR) for MR measurements,or to reduce motion and/or flow sensitivity of the measurement, it isstandard for multiple acquisitions be executed so as to acquire themeasurement data multiple times, in order to be able to average themeasurement data or for example the reconstructed image data.

The desire for ever faster MR acquisitions in the clinical environmentis currently resulting in a renaissance for methods with which multipleimages are acquired simultaneously. Generally such methods can becharacterized by the fact that, at least during part of the measurementprocedure, the transverse magnetization of at least two slices is usedsimultaneously for the imaging process (multi-slice imaging, slicemultiplexing). In contrast to established multislice imaging, thesignals from at least two slices are acquired alternately, in otherwords completely independently of one another, with a correspondinglylonger measurement time.

An example of known methods for this purpose are Hadamard encoding,methods using simultaneous echo refocusing, methods using wideband dataacquisition, and methods deploying parallel imaging in the slicedirection. The last-mentioned methods include, for example, theCAIPIRINHA technique, as described by Breuer et al. in “ControlledAliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA)for Multi-Slice Imaging”, Magnetic Resonance in Medicine 53, 2005, pp684-691, and the blipped CAIPIRINHA technique, as described by Setsompopet al. in “Blipped-Controlled Aliasing in Parallel Imaging forSimultaneous Multislice Echo Planar Imaging With Reduced g-FactorPenalty”, Magnetic Resonance in Medicine 67, 2012, pp 1210-1224.

With the last-mentioned slice multiplexing methods, a multiband RF pulseis used to excite or otherwise manipulate, for example to refocus orsaturate, two or more slices simultaneously. Such a multiband RF pulseis a multiplex (superimposition) of individual RF pulses, which would beused to manipulate the single slices to be manipulated simultaneously.Multiplexing produces a baseband-modulated multiband RF pulse from theadded pulse forms of the individual RF pulses. Spatial encoding of theacquired signals is achieved by standard gradient switching in twodirections (two-dimensional gradient encoding).

The resulting signals are acquired using multiple reception antennas,and are collapsed from all the excited slices in one k-space dataset andthen separated for the single slices with the use of parallelacquisition reconstruction techniques.

The cited parallel acquisition techniques are used to shorten theacquisition times. The acquisition times generally require to acquirethe desired data use an incomplete (with respect to the Nyquistcriterion) sampling, in other words an undersampling. The parallelacquisition techniques include, for example, GRAPPA (“GeneRalizedAutocalibrating Partially Parallel Acquisition”) and SENSE (“SENSitivityEncoding”). The measurement points in k-space that are not filled by adata entry in the undersampling are generally regularly distributed overk-space in parallel acquisition techniques, so that, for example, everyother k-space line is filtered. The “missing” k-space data stillcontribute to the reconstruction with the use of coil sensitivity datain parallel acquisition techniques. This coil sensitivity data for thereception coils that are used when acquiring the measurement data isdetermined from reference measurement data, which samples at least oneregion of k-space, generally the central region, completely according tothe Nyquist criterion.

With slice multiplexing methods, parallel acquisition techniques areused to separate again the measurement data entered in k-space, whichwere acquired simultaneously for different slices. Reference measurementdata must be acquired for all the slices involved. This is generallydone in an additional reference measurement, which detects referencemeasurement data individually for each desired slice.

In order to be able to separate the resulting signals acquired from thedifferent slices, a different phase is assigned to each of theindividual RF pulses before multiplexing, for example by adding a phaseamount, which increases linearly (e.g. with k-space coordinate in thephase encoding direction (k_(y))). This allows each slice to be assigneda different phase increase, with the result that the slices aredisplaced in relation to one another in the image domain. This shift iscontrolled by the FOV (field of view) shift factor. DE102016218955,published later, describes how an optimum FOV shift factor can bedetermined.

In the CAIPIRINHA methods described in the cited articles by Breuer etal. and Setsompop et al., switching additional gradient blips oradditional modulation of the phases of the RF pulses causes themultiband RF pulses to be assigned further phase shifts that alternatebetween the simultaneously excited slices, which generate shifts in theimage domain. These additional shifts in the image domain improve thequality of the separation of the signals from the slices, particularlyif the coil sensitivities demonstrate such small differences in thesensitivity profiles of the individual coils so that they are notsufficient to separate the slices reliably. This reduces artifacts inthe image data ultimately reconstructed from the measured measurementdata.

FIG. 1 contrasts examples of different sampling schemes for k-space forGRAPPA-type parallel imaging techniques, in each instance withacceleration factor 2 and a three-dimensional (3D) Cartesian samplingscheme, which in each instance samples k-space lines in the k_(y)-k_(x)plane. The k_(x) direction, in which the illustrated k-space lines runin the example, is perpendicular to the plane of the drawing sheet, andthe sampling scheme is always the same in the k_(x) direction. The solidcircles represent measured k-space points, the empty circles representomitted (unfilled) k-space points. The left-side sampling scheme showsconventional GRAPPA sampling, where every other k-space line in onespatial direction (here the k_(z) direction) is omitted, and thus onlyhalf of the points in k-space that are available to be filled with adata entry are actually filled.

The effect of the additional phase shifts on the sampling scheme of atwo-dimensional (2D) slice multiplexing measurement can be described asillustrated in FIG. 1 on the right. The additional phases assigned inslice multiplexing CAIPIRINHA (Controlled Aliasing in Parallel ImagingResults in Higher Acceleration) technique methods cause the measurementpoints subject to the additional phase to be displaced by a shift ink-space in the k_(z) direction. The size of this shift in the k_(z)direction is a function of the assigned phase. This is also described inthe article by Zahneisen et al.: “Three-Dimensional Fourier Encoding ofSimultaneously Excited Slices: Generalized Acquisition andReconstruction Framework”, Magn. Reson. Med. 71, pp 2071-2081 (2014).

The reference data, from which the sensitivity data are obtained,conventionally must be measured additionally for every SMS measurement.The additional acquisition of the reference measurement data increasesthe acquisition time required overall and the SAR exposure (SAR:Specific Absorption Rate) when using a slice multiplexing method, andthus reduces the advantages of reduced measurement time and SAR exposurecompared with single-slice methods, which are actually the goal of suchmethods.

Deviations in the measurement parameters used to acquire the referencemeasurement data from the measurement parameters used for multibandacquisition, in particular measurement parameters relating to theproperties of the RF excitation pulses and/or relating to the read-outoperation, for example the read-out bandwidth, can also influence thequality of the separation of the slices and produce unwanted artifacts.In this respect the U.S. patent application Ser. No. 15/262,233,published later, describes different methods for obtaining suchreference measurement data in addition to the multiband measurementdata, the reference data still having to be acquired in addition to themeasurement data.

SUMMARY OF THE INVENTION

An object of the invention is to avoid the disadvantages resulting fromthe additional acquisition of reference measurement data as well as theacquisition of the multiband measurement data with MR slice multiplexingmethods, without reducing the quality of the measurement data obtained,which has been separated for single slices.

In the inventive method for generating measurement data for at least twonon-overlapping slices of an examination object by operation of amagnetic resonance scanner with a measurement sequence, measurement dataare entered at points on a k-space trajectory in a memory organized ask-space, at least one multiband RF excitation pulse is radiated thatexcites the at least two non-overlapping slices simultaneously andselectively. Also in the invention method, phases are determined in acomputer which are to be additionally assigned to each of thesimultaneously excited slices during the course of the execution of themeasurement sequence, so that at least a first of the measured k-spacepoints is displaced in a k-space direction away from the originalk-space trajectory of the measurement sequence according to theadditionally assigned phases. The measurement data are acquired using aleast two reception coils based on the measurement sequence, subject toassignment of the determined additional phases. The acquisition ofmeasurement data with the at least two reception coils is repeated basedon the measurement sequence, the phases to be additionally assignedbeing changed compared with the previous acquisition of measurement datasuch that at least a second of k-space points measured by themeasurement sequence, not yet displaced away from the original k-spacetrajectory in the previously performed acquisitions, is displaced awayfrom the original k-space trajectory in k-space direction. Theacquisition of the measurement data is repeated, subject to thisadjustment of the additional phases in each repetition, as many times asare necessary in order for at least one central k-space region to besampled (filled) completely by the repeated acquisitions. The computerdetermines a calibration dataset from the measurement data acquired inthis central k-space region. The computer reconstructs image data forthe simultaneously excited slices from the acquired measurement datausing the calibration dataset, and makes the image data available inelectronic form, as a data file.

In the inventive slice multiplexing method, therefore, measurements areperformed repeatedly subject to the assignment of additional phases, theadditionally assigned phases being changed with reach repetition suchthat at least one central k-space region is sampled completely by therepeated acquisitions. A calibration dataset is determined from themeasurement data acquired completely in the central k-space region, thiscalibration dataset being used when reconstructing image data for thesimultaneously excited slices from the acquired measurement data. Thereis thus no need for the additional calibration measurements that areconventionally necessary, in order to determine calibration data withthe inventive method.

If measurements are planned to be performed repeatedly, such as toimprove, by averaging, the quality of the obtained measurement and/orimage data, the measurement time with the inventive method is notextended by calibration measurements.

With the inventive method, calibration data are determined from the samemeasurement data, from which the desired image data are alsoreconstructed. The measurement parameters of the measurement data, fromwhich the calibration data are determined, and the measurementparameters of the measurement data, from which image data arereconstructed, are therefore identical, with the result that thecalibration data are inherently matched to the measurement data, whichis to be supplemented using the calibration data. This effectivelyprevents artifacts.

An inventive magnetic resonance has an MR data acquisition scanner thathas a basic field magnet, a gradient coil arrangement, a radio-frequencyantenna and a control computer with a radio-frequency transmit/receivecontroller with a multiband RF pulse unit configured to perform theinventive method.

The present invention also encompasses a non-transitory,computer-readable data storage medium encoded with programminginstructions that, when the storage medium is loaded into a controlcomputer or computer system of a magnetic resonance apparatus, cause thecomputer or computer system to operate the magnetic resonance apparatusin order to implement any or all embodiments of the method according tothe invention, as described above.

The advantages and embodiments cited in relation to the method apply ina similar manner to the magnetic resonance apparatus and theelectronically readable data storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic comparison of different k-space sampling schemesfor parallel acquisition techniques.

FIG. 2 is a flowchart of the inventive method.

FIG. 3 schematically illustrates an example of inventive k-spacesampling.

FIG. 4 schematically illustrates another example of inventive k-spacesampling.

FIG. 5 schematically illustrates the inventive magnetic resonancesystem.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a flowchart of the inventive method for generating measurementdata for at least two non-overlapping slices Sn (n≥2) of an examinationobject by magnetic resonance technology.

The measurement data are acquired here based on a measurement sequenceSeq, which determines measurement points on a k-space trajectory, andincludes at least one multiband RF excitation pulse, which excites theat least two non-overlapping slices Sn simultaneously and selectively.

The measurement sequence Seq determines the RF pulses to be essentiallyradiated, gradients to be switched, and read-out operations, as well astheir time sequence. This therefore determines k-space trajectory, alongwhich k-space is sampled during acquisition of the measurement points.

The measurement sequence Seq can be a standard two-dimensionalmeasurement sequence for the acquisition of measurement data in slices,in which k-space trajectory determined by the measurement sequence liesin a plane in k-space. K-space direction, in which measurement pointsare displaced by assigning the additional phase, can then beperpendicular to this plane of k-space trajectory, in order to be ableto cover the multiple slices. Such selection of the measurement sequenceSeq facilitates implementation of the inventive method on magneticresonance systems, as such measurement sequences are generally alreadyavailable and it is thus possible to access existing protocols, inparticular those of CAIPIRINHA methods.

Phases to be additionally assigned ϕ₁ are determined in a computer, thedetermined phases being assigned to each of the simultaneously excitedslices Sn during the course of a (first) execution of the measurementsequence Seq (i=1) (Block 102), the index i counting the repetitions ofthe acquisition of measurement data using the measurement sequence Seq.The assigning of the additional phases ϕ₁ means that at least a first ofthe measured k-space points is displaced away from the original k-spacetrajectory of the measurement sequence Seq in a k-space direction in thearea of k-space that is sampled as part of an acquisition of measurementdata using the measurement sequence Seq, and subject to assignment ofthe determined additional phases ϕ₁. A specific additional phase can bedetermined for each of the simultaneously excited slices Sn, so that notwo of the simultaneously excited slices Sn are assigned the sameadditional phase ϕ₁.

The k-space direction, in which the at least one k-space point ofk-space trajectory is displaced, can correspond to the slice direction(k_(z)) in k-space.

The additional phases ϕ₁ can be assigned, for example, by additionalgradients to be switched and/or by manipulation of the phases of thesingle RF pulses of the multiband RF excitation pulses used. Inparticular, the additional phases ϕ₁ can be assigned according to aCAIPIRINHA method.

To this end, a desired shift in the image space can be determined (Block101) and the determined phases to be additionally assigned ϕ₁ can bedetermined such that they introduce the desired shift in the image spaceinto the acquired measurement data MD₁.

A desired shift in the image space can be determined by the computer foreach of the slices Sn, by which, in each instance, one slice of thesimultaneously excited slices Sn is to be displaced in the image domain,after a transformation of the measurement data MD₁ containing all theslices Sn into the image domain, so that the slices Sn in the imagedomain are each displaced relative to one another. The phases ϕ₁ to beadditionally assigned, determined for each of the simultaneously excitedslices Sn, are determined so as to produce the desired respective shiftin the image domain for each of the simultaneously excited slices Sn inthe acquired measurement data MD₁ containing all the simultaneouslyexcited slices Sn.

The desired shift in the image space can be determined, for example,based on the number N of simultaneously excited slices Sn. This can bedone by each of the simultaneously excited slices Sn being shifted by amultiple of the Nth part of the field of view FOV, for example by

$( {n - 1} ){\frac{FOV}{N}.}$

As described in the article by Zahneisen et al. cited above, such aselection of the desired shift in the image domain results in a shift ofthe measured k-space points in k-space (in the k_(z) direction) by anamount that corresponds to complete sampling according to Nyquist inthat k-space direction. Thus a 2D slice multiplexing measurement can beconsidered to be the same as a 3D CAIPIRINHA measurement.

The desired shift in the image space, however, can also take placeaccording to a method in DE102016218955 cited above.

With the method described in FIG. 2 measurement data MD₁ are acquired aspart of a first acquisition using at least two reception coils based inthe measurement sequence Seq subject to assignment of the determinedadditional phases ϕ₁ (Block 103).

The acquisition of measurement data with the at least two receptioncoils based on the measurement sequence Seq is repeated (Block 105),with the result that further measurement data MD_(i) is acquired. Withsuch repeated acquisition of measurement data (i≥2) phases to beadditionally assigned ϕ_(i) are changed compared with the previousacquisition of measurement data with index j<i in such that at least asecond of k-space points measured by the measurement sequence Seq, notyet displaced away from the original k-space trajectory in thepreviously performed acquisitions, is displaced away from the originalk-space trajectory in k-space direction. A determination of the phasesto be additionally assigned ϕ_(i) to be used for a repetition (Block104) can take place by taking into account k-space points alreadydisplaced at least once in previous acquisitions. The determination ofthe phases to be additionally assigned, which are to be used for aparticular repetition, can additionally or alternatively take intoaccount which k-space points have not yet been acquired for the desiredcomplete sampling at least in the central k-space region. This keeps thenumber of repetitions to be performed as low as possible.

One example of a determination of a phase ϕ₁ additionally assigned for arepetition is described with reference to FIG. 3, in which the principleof k-space sampling is illustrated as an example with a field of viewregion shift factor of

$\frac{FOV}{2}$for a first acquisition (i=1) and a repetition of the acquisition (i=2).Here, as in FIG. 1, the k_(x) direction is perpendicular to the plane ofthe paper. FIG. 3 therefore shows a Cartesian sampling scheme. Themethod can also be applied in a similar manner for non-Cartesiansampling schemes, for example radial or spiral sampling schemes.

In FIG. 3, as in FIG. 1, acquired k-space points are shown as blackcircles, while k-space points that have not been acquired are shown aswhite circles. In the first acquisition shown on the left every otherk-space point is displaced in k_(y) direction by additionally assignedphases ϕ₁ in k_(z) direction. This can be done, for example, by applyinga CAIPIRINHA technique. In the repetition shown on the right theadditional phases ϕ_(i) are changed in such a manner that every otherk-space point is shifted in the k_(y) direction again by additionallyassigned phases ϕ₂ in k_(z) direction but in the repetition k-spacepoints that were not displaced in the first acquisition are displaced ink_(z) direction. In the illustrated example it is therefore possible toachieve the desired complete k-space sampling even for the entirek-space by simply swapping k-space points populated with additionalphases ϕ_(i) in repetitions of the acquisition.

Such swapping of k-space points subject to additional phases can takeplace in the same manner for other acceleration factors and/or field ofview shift factors, the number of repetitions required for an, at leastpartially, complete sampling being a function of the accelerationfactors and field of view shift factors used.

For example, with a field of view shift factor of

$\frac{FOV}{3}$and an otherwise similar sampling scheme the desired k-space regioncould be sampled completely in three repetitions, in which k-spacepoints populated with additionally assigned phases ϕ_(i) arerespectively permuted.

A further example of swapping k-space points subject to the additionalphases is shown in FIG. 4. This shows a sampling scheme for a field ofview shift factor of

$\frac{FOV}{4},$with which the phases to be additionally assigned are distributed over atotal of four repetitions of the acquisition of the measurement data insuch a manner that the desired k-space region is sampled completely bythe four acquisitions. In FIG. 4 k-space points measured in oneacquisition are shown as identically filled in circles. For example in afirst acquisition k-space points shown as black circles are measured, ina second acquisition k-space points shown as circles with a stripe frombottom left to top right are measured, in a third acquisition k-spacepoints shown as circles with horizontal stripes are measured and in afourth acquisition k-space points shown as white circles are measured.

With each repetition of the acquisition of measurement data the use ofthe same measurement sequence means that the same number of k-spacepoints is sampled each time.

Such an acquisition of measurement data MD_(i) with respectivelyadjusted phases to be additionally assigned ϕ_(i) is repeated so oftenand with phases to be additionally assigned ϕ_(i) that are adjusted insuch a manner in each instance that at least a central k-space region issampled completely according to Nyquist as part of the repeatedacquisitions (Query 107).

If the measurement sequence is selected in such a manner that k-spacetrajectory determined by the measurement sequence allows a completesampling of k-space according to Nyquist, fewer repetitions are neededto sample the central k-space region completely.

To this end a total measurement dataset gzMD is formed from thepreviously acquired measurement data MD_(i), containing at least all thepreviously measured measurement data in a predefined central k-spaceregion, or even all the previously measured measurement data MD_(i)(Block 106).

If the answer to Query 107 is that the central k-space region has notyet been sampled completely by the previously performed repetitions(“n”), a new repetition is started with new phases to be additionallyassigned ϕ_(i) (Block 104).

If the answer to Query 107 is that the central k-space region hasalready been sampled completely by the previously performed repetitions(“y”), a calibration dataset CD is determined from the completemeasurement data acquired in the central k-space region (Block 108).

An acquisition calibration dataset CD_(i) can be determined for each orat least two of the items of measurement data MD_(i) acquired as part ofone acquisition, different k-space points of the actual k-spacetrajectory being displaced in k-space direction therein, therespectively displaced k-space points which are therefore missing fromthe original k-space trajectory being able to be reconstructed withthis. The calibration dataset CD can thus contain multiple acquisitioncalibration datasets CD_(i).

Image data BD of the simultaneously excited slices Sn are reconstructedfrom the acquired measurement data MD_(i) (i=1, 2, . . . ) using thecalibration dataset CD (Block 110). At least one image dataset BD isreconstructed here for each of the simultaneously excited slices Sn.

Reconstruction of the image data involves separation of the acquiredmeasurement data into at least one slice measurement dataset SMD foreach simultaneously excited slice Sn (Block 109).

Such separation of the acquired measurement data can be performed basedon a parallel acquisition technique, for example a slice GRAPPA method,as described in the article by Breuer et al. cited above, using thecalibration dataset CD. Such separation can take place separately forthe measurement data MD_(i) acquired in each instance as part of oneacquisition, for example with an associated acquisition calibrationdataset CD_(i), allowing associated slice measurement datasets SMD to begenerated for each simultaneously excited slice Sn for each acquisition.This allows multiple slice measurement datasets SMD to be generated foreach simultaneously excited slice. The phases ϕ₁ to be additionallyassigned, which have been adjusted with each repetition and thereforechanged, increase the variation of the additional phases used in theacquisitions, which already improves the robustness of the separation ofthe slices by means of parallel acquisition techniques.

If the stored total measurement dataset gzMD in k-space direction, inwhich k-space points are displaced by the additional phases ϕ_(i), e.g.the slice direction k_(z), is complete according to Nyquist, anassociated slice measurement dataset SMD can be extracted from the totalmeasurement dataset by applying a Fourier transform in the directioncorresponding to the shift for each excited slice and then stored.

No calibration data are required here to separate the measurement datainto slice measurement data. The calibration data can be used tosupplement missing measurement data in the plane of k-space trajectoryof the measurement sequence, for example to supplement measurement dataundersampled in the phase encoding direction.

Reconstruction of the image data can involve separating the acquiredmeasurement data into at least two slice measurement datasets SMD foreach simultaneously excited slice Sn. For example a slice measurementdataset SMD can be separated out from multiple measurement data MD_(i)acquired for each acquisition for each of the simultaneously excitedslices Sn.

The slice measurement datasets SMD themselves can be incompleteaccording to Nyquist. K-space points that are missing according toNyquist can then be reconstructed using a parallel acquisition techniqueand the calibration dataset CD and supplemented in the slice measurementdatasets. The reconstruction of the image data can thus includes suchsupplementing of incomplete slice measurement datasets SMD.

If multiple slice measurement datasets SMD have been determined for eachsimultaneously excited slice Sn, the reconstruction of the image data BDcan include an averaging of at least two of the slice measurementdatasets SMD of at least one of the simultaneously excited slices Snrespectively to provide an averaged slice measurement dataset of therespective slice.

Additionally or alternatively, reconstruction of the image data BD caninclude an averaging of the image data reconstructed from multiple slicemeasurement datasets SMD of a slice for the respective slice.

Such averaging improves the quality of the image data ultimatelyobtained, by reducing sensitivity to motion of the examination object,for example also respiratory motion or cardiac pulse motion, andimproves the SNR.

Reconstruction of the image data can also include measures to compensatefor undesirable phase offsets that may possibly occur due to theadditionally assigned phases.

FIG. 5 schematically shows an inventive magnetic resonance (MR)apparatus 1. The MR apparatus 1 has an MR data acquisition scanner witha basic field magnet 3 that generates the constant magnetic field, agradient coil arrangement 5 that generates the gradient fields, aradio-frequency antenna 7 for radiating and receiving radio-frequencysignals, and a control computer 9 configured to perform the inventivemethod. In FIG. 5 such sub-units of the magnetic resonance apparatus 1are only outlined schematically. The radio-frequency antenna 7 iscomposed of multiple sub-units, in particular at least two coils, forexample the schematically shown coils 7.1 and 7.2, which can beconfigured either only to transmit radio-frequency signals or only toreceive the triggered radio-frequency signals (MR signals), triggered orto do both.

In order to acquire MR data from an examination object U, for example apatient or a phantom, the examination object U is introduced on a bed Linto the measurement volume of the scanner. The slices S1 and S2 areexamples of two different slices of the examination object, from whichMR data can be acquired simultaneously.

The control computer 9 centrally controls the magnetic resonanceapparatus, and can control the gradient coil arrangement 5 with agradient controller 5′ and the radio-frequency antenna 7 with aradio-frequency transmit/receive controller 7′. The radio-frequencyantenna 7 has multiple channels, in which signals can be transmitted orreceived.

The radio-frequency antenna 7 together with its radio-frequencytransmit/receive controller 7′ is responsible for generating andradiating (transmitting) a radio-frequency alternating field formanipulating the nuclear spins in a region to be examined (in particularin different slices S1 and S2) of the examination object U. The centerfrequency of the radio-frequency alternating field, also referred to asthe B1 field, here should be close to the resonant frequency of thenuclear spins to be manipulated. To generate the B1 field, currentscontrolled by the radio-frequency transmit/receive controller 7′ areapplied to the RF coils in the radio-frequency antenna 7.

The control computer 9 also has a phase determination processor 15 thatdetermines phases ϕ₁ to be additionally assigned according to theinvention.

A computation processor 13 of the control computer 9 is configured toexecute all the computation operations required for the requiredmeasurements and determinations. Intermediate results and final resultsrequired for this purpose or determined in the process can be stored ina memory S of the control computer 9. The units shown here should notnecessarily be considered as physically separate units, but simplyrepresent a subdivision into functional units, which can also beimplemented by fewer physical units, or just one.

A user can enter control commands into the magnetic resonance apparatus1 and/or view displayed results, for example image data, from thecontrol computer 9 via an input/output interface E/A.

A non-transitory, data storage medium 26 can be loaded into the controlcomputer 9, and is encoded with programming instructions (program code)that cause the control computer 9, and the various functional unitsthereof described above, to implement any or all embodiments of themethod according to the invention, as also described above.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the Applicant to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of the Applicant's contribution to theart.

The invention claimed is:
 1. A method for magnetic resonance (MR)imaging of at least two non-overlapping slices of an examination object,said method comprising: from a computer, emitting control signals to anMR data acquisition scanner in order to cause the MR data acquisitionscanner to execute an MR data acquisition sequence in which acquired MRdata are entered into a memory organized as k-space along a k-spacetrajectory, and in order to radiate at least one multibandradio-frequency (RF) excitation pulse that selectively andsimultaneously excites nuclear spins in at least two non-overlappingslices of an examination subject situated in the MR data acquisitionscanner; in said computer, determining phases that are to beadditionally assigned to each of the simultaneously excited slicesduring execution of said MR data acquisition sequence, so as to displaceat least a first of the measurement points for a respective slice to bedisplaced away from said k-space trajectory of the MR data acquisitionsequence, in a direction in k-space corresponding to the additionallyassigned phase for that respective slice; using said computer to operatesaid MR data acquisition scanner to execute said MR data acquisitionsequence with the assignment of the determined additional phases inorder to acquire MR data from the respective slices with at least two RFreception coils; using said computer to emit control signals to operatesaid MR data acquisition scanner in order to repeat acquisition of saidMR data with said at least two RF reception coils in said MR dataacquisition sequence with the phases that are additionally assignedbeing changed from repetition-to-repetition so that, compared with aprevious acquisition of said MR data, at least a second of themeasurement points in a respective repetition of the MR data acquisitionsequence, that have not yet been displaced away from said k-spacetrajectory in previous repetitions, are displaced away from k-spacetrajectory in said direction in k-space, with said repetitions of saidMR data acquisition sequence being repeated with a number of repetitionsthat causes at least one central region of k-space to be sampledcompletely by the repeated MR data acquisitions; in said computer,determining a calibration dataset from the measurement data acquired insaid central region of k-space; and in said computer, reconstructingimage data for the simultaneously excited slices from the acquiredmeasurement data using said calibration dataset, and making the imagedata available from the computer in electronic form, as a data file. 2.A method as claimed in claim 1 comprising, in said computer, determininga different phase that is to be additionally assigned for each of thesimultaneously excited slices.
 3. A method as claimed in claim 1comprising assigning the additional phases to the respective slices byemitting control signals from said computer to said data acquisitionscanner that cause at least one of activation of additional gradients,and manipulation of phases of individual RF pulses of which saidmultiband RF excitation pulse is comprised.
 4. A method as claimed inclaim 1 comprising assigning said additional phases according to aCAIPIRINHA method.
 5. A method as claimed in claim 1 comprisingdetermining said k-space trajectory so as to be in a plane in k-space,and determining said direction in k-space, in which said measurementpoints are displaced by the assigned additional phase, so as to beperpendicular to the plane of said k-space trajectory.
 6. A method asclaimed in claim 1 comprising, in said computer, determining apredetermined shift in the image domain, in which said image data exist,and determining said phases to be additionally assigned so as to producesaid predetermined shift in the image domain by the acquired measurementdata.
 7. A method as claimed in claim 6 comprising determining saidpredetermined shift in the image domain based on a number of said slicesfrom which said MR data are simultaneously acquired.
 8. A method asclaimed in claim 1 comprising, when reconstructing said image data,separating the acquired measurement data in k-space into at least oneslice measurement dataset for each of the slices from which said MR datawere simultaneously acquired.
 9. A method as claimed in claim 8comprising implementing said separation using a parallel acquisitiontechnique.
 10. A method as claimed in claim 8 comprising entering theacquired MR data into k-space so that each slice measurement dataset isacquired in said direction of k-space, in which said measurement pointsare displaced by said additional phase, is complete according to theNyquist criterion and, in said computer, extracting an associated slicemeasurement dataset from the respective slice measurement dataset byapplying a Fourier transform thereto in a direction corresponding tosaid shift.
 11. A method as claimed in claim 8 comprising reconstructingsaid image data by separating the acquired measurement data into atleast two slice measurement datasets for each of said slices, andaveraging at least two of said slice measurement datasets of at leastone slice, or averaging the image data reconstructed from the slicemeasurement datasets of at least one slice.
 12. A magnetic resonance(MR) apparatus comprising: an MR data acquisition scanner; a computerconfigured to emit control signals to said MR data acquisition scannerin order to cause the MR data acquisition scanner to execute an MR dataacquisition sequence in which acquired MR data are entered into a memoryorganized as k-space along a k-space trajectory, and in order to radiateat least one multiband radio-frequency (RF) excitation pulse thatselectively and simultaneously excites nuclear spins in at least twonon-overlapping slices of an examination subject situated in the MR dataacquisition scanner; said computer being configured to determine phasesthat are to be additionally assigned to each of the simultaneouslyexcited slices during execution of said MR data acquisition sequence, soas to displace at least a first of the measurement points for arespective slice to be displaced away from said k-space trajectory ofthe MR data acquisition sequence, in a direction in k-spacecorresponding to the additionally assigned phase for that respectiveslice; said computer being configured to operate said MR dataacquisition scanner to execute said MR data acquisition sequence withthe assignment of the determined additional phases in order to acquireMR data from the respective slices with at least two RF reception coils;said computer being configured to emit control signals to operate saidMR data acquisition scanner in order to repeat acquisition of said MRdata with said at least two RF reception coils in said MR dataacquisition sequence with the phases that are additionally assignedbeing changed from repetition-to-repetition so that, compared with aprevious acquisition of said MR data, at least a second of themeasurement points in a respective repetition of the MR data acquisitionsequence, that have not yet been displaced away from said k-spacetrajectory in previous repetitions, are displaced away from k-spacetrajectory in said direction in k-space, with said repetitions of saidMR data acquisition sequence being repeated with a number of repetitionsthat causes at least one central region of k-space to be sampledcompletely by the repeated MR data acquisitions; said computer beingconfigured to determine a calibration dataset from the measurement dataacquired in said central region of k-space; and said computer beingconfigured to reconstruct image data for the simultaneously excitedslices from the acquired measurement data using said calibrationdataset, and to make the image data available from the computer inelectronic form, as a data file.
 13. A non-transitory, computer-readabledata storage medium encoded with programming instructions, said storagemedium being loaded into a computer of a magnetic resonance (MR)apparatus that comprises an MR data acquisition scanner, and saidprogramming instructions causing said computer system to: emittingcontrol signals to said MR data acquisition scanner in order to causethe MR data acquisition scanner to execute an MR data acquisitionsequence in which acquired MR data are entered into a memory organizedas k-space along a k-space trajectory, and in order to radiate at leastone multiband radio-frequency (RF) excitation pulse that selectively andsimultaneously excites nuclear spins in at least two non-overlappingslices of an examination subject situated in the MR data acquisitionscanner; determine phases that are to be additionally assigned to eachof the simultaneously excited slices during execution of said MR dataacquisition sequence, so as to displace at least a first of themeasurement points for a respective slice to be displaced away from saidk-space trajectory of the MR data acquisition sequence, in a directionin k-space corresponding to the additionally assigned phase for thatrespective slice; operate said MR data acquisition scanner to executesaid MR data acquisition sequence with the assignment of the determinedadditional phases in order to acquire MR data from the respective sliceswith at least two RF reception coils; emit control signals to operatesaid MR data acquisition scanner in order to repeat acquisition of saidMR data with said at least two RF reception coils in said MR dataacquisition sequence with the phases that are additionally assignedbeing changed from repetition-to-repetition so that, compared with aprevious acquisition of said MR data, at least a second of themeasurement points in a respective repetition of the MR data acquisitionsequence, that have not yet been displaced away from said k-spacetrajectory in previous repetitions, are displaced away from k-spacetrajectory in said direction in k-space, with said repetitions of saidMR data acquisition sequence being repeated with a number of repetitionsthat causes at least one central region of k-space to be sampledcompletely by the repeated MR data acquisitions; determine a calibrationdataset from the measurement data acquired in said central region ofk-space; and reconstruct image data for the simultaneously excitedslices from the acquired measurement data using said calibrationdataset, and make the image data available from the computer inelectronic form, as a data file.